Ochem4 Alkanes Flashcards
In general, as the molecular weight of an alkane increases, what happens to the MP, BP, and density?
Branched compounds have slightly lower or higher BPs and MPs than their straight chain isomers? Why?
They all increase
Lower
Reduces surface area and decreases intermolecular attractive forces (Van der Waals). Branched structures are also more difficult to pack into tight 3D structures
A frequently encountered reaction of alkanes in which one or more hydrogen atoms are replaced by halogen atoms
Halogenation
In the first step of halogenation, diatomic halogens are cleaved by heat or light, resulting in the formation of
Free radicals
ex: X2 —> 2X`
(2X` is two free radical halogens)
The second step of halogenation is one in which a radical produces another radical that continues the reaction. A halogen free radical reacts with an ______ removing a hydrogen atom to form HX and an alkyl radical
alkene
X + RH ---> HX + R
The final step of halogenation occurs when two free radicals combine with one another to form a stable molecule.
X + R = RX
For halogenation reactions, bromine generally attacks the hydrogen that is bonded to the carbon that can form the most ______ free radical
What is the trend, then, of likeliness of substituted alkanes to be formed into a radical during free-radical bromination?
Stable
3°>2°>1°>methyl
How are chlorination reactions different from bromination?
What does it depend upon?
What are they most likely to react with?
Do chlorination reactions produce mixtures if there are multiple options?
It is more rapid
Stability of intermediate and the number of hydrogens present.
Hydrogens of primary hydrogens because of their abundance
Yes. Therefore, chlorination works best if there is only one type of hydrogen.
The reaction of alkanes with molecular oxygen to form CO2, H2O, and heat is called
Is the reaction often complete?
Combustion
No, lots of CO is produced (carbon monoxide)
This occurs when a molecule is broken down by heat
It’s also called
What does it form when used on alkanes?
Pyrolysis
Cracking
Free radicals
Ex: CH3CH2CH3 —> CH3 CH2CH3
The alternative to pyrolysis, occurs when a radical transfers a hydrogen atom to another radical, producing an alkane and an alkene
CH3 + CH2CH3 —> CH4 + CH2=CH2
Disproportionation
If nucleophiles have the same attacking atom, then nucleophiliciy is roughly correlated to ____
In other words the stronger the ___, the stronger the nucleophile
basicity
Base
Rank these on nucleophilic strength
OH-, RCO2-, ROH, H2O, RO-,
RO->HO->RCO2->ROH>H2O
Rank them by basicity
In protic solvents, larger or smaller atoms are better nucleophiles?
Rank these molecules in terms of nucleophilic strength in protic solvents
I-, CN-, H2O, OH-, Br-, Cl-, F-, RO-
Larger atoms, they can shed their solvent molecules and are more polarizable.
CN->I->RO->HO->Br->Cl->F->H2O
In a aprotic solvents, what determines the effectiveness of nucleophiles?
Rank these molecules in terms of nucleophilic strength in aprotic solvents
Basicity
F->Cl->Br->I-
Opposite of polar solvents!
The best leaving groups in nucleophilic substitution are those that are
Rank these halogens in terms of leaving groups:
Br, Cl, F, I
Weak Bases
I>Br>Cl>F
A substitution reaction where the rate of the reaction is dependent upon only one species
What is generally the rate determining step in these reactions?
SN1
Dissociation to form a stable, positively charged ion called a carbocation
In the formation of a carbocation in Sn1 reactions, what are more stable carbocations?
Rank methyl, primary, secondary and tertiary carbocations on stability
So which favor Sn1 reactions the most? The least?
More substituted
Tertiary>Secondary>Primary>methyl
Tertiary the most, methyl the least
Do Sn1 reactions favor polar or apolar solvents? Why?
What type of leaving groups are best?
Polar, as polar solvent are better at surrounding and isolating ions. So polar protic solvents work best
Weak bases (I-, etc)
A substitution reaction that has 2 molecules in its rate determining step, a substrate and nucleophile
The nucleophile must be strong and can’ be sterically hindered
Sn2 Reaction
What type of stereochemistry is produced for Sn1 reactions
What about Sn2?
Racemic mixtures
Inversion of stereochemistry
Which reaction favors polar aprotic solvents, Sn1 or Sn2?
Which reaction favors polar protic solvents?
Sn2
Sn1
Which reaction favors tertiary carbons, Sn1 or Sn2?
methyl groups?
Sn1
Sn2
Which reaction is favored with the use of bulky nucleophiles, Sn1 or Sn2?
Sn1
